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From Wikipedia:
The grizzly bear (Ursus arctos horribilis) population within the Yellowstone ecosystem is estimated to be approximately 280-610 bears. The average lifespan of a grizzly bear in Yellowstone is about 22 years, and the average lifespan of a Yellowstone black bear is about 17.

Bears were once commonly observed along roadsides and within developed areas of Yellowstone National Park. Bears were attracted to these areas by the availability of human foods in the form of handouts and unsecured camp groceries and garbage. Although having bears readily visible along roadsides and within developed areas was very popular with the park visitors, it was also considered to be the primary cause of an average of 48 bear-caused human injuries per year from 1930 through 1969. Rocky Mountain grizzly bears have killed several people in the park since the 1970s.

In 1970, the park took a gamble and initiated an intensive bear management program. The objectives of this change was restoring the grizzly bear and black bear populations to subsistence on natural forage and reducing bear-caused injuries to humans. As part of the bear management program implemented in 1970, regulations prohibiting the feeding of bears were strictly enforced. Some experts believed that the bears would not survive this change. As the bears became more desperate for food and more aggressive, more of them were encountered in camps and in the next several years, over 100 grizzly bears had to be euthanized, putting the park bear population on the brink of extinction. On July 28, 1975, under the authority of the Endangered Species Act, the United States Fish and Wildlife Service listed the grizzly bear in the lower 48 states as a threatened species.

Over the next several decades, the bears learned to hunt and forage for themselves from non-human food sources, and their population slowly grew. On March 22, 2007, grizzly bears were taken off of the endangered species list. In the 30 years since the grizzly was listed as a threatened species, the Yellowstone population increased from 126 to 500. "The grizzly is a large predator that requires a great deal of space, and conserving such animals is a challenge in today's world," Deputy Interior Secretary Lynn Scarlett said in announcing the decision. "I believe all Americans should be proud that, as a nation, we had the will and the ability to protect and restore this symbol of the wild."